You can do the "font size change" with javascript.
With the email templates, you can add the HTML to make the header and footer display.
In the email template text box (for the reply to the customer) I expect you would type in something like <img src="http://www.yoursite.com/images/email_header.jpg"> and repeat for the footer in the places you want these to appear (top and bottom of the mesage) with the {element_x} or {form_data} bits in between; you can probably test this to make sure it works by filling in the form so Machform will respond to your own email.
Since the emails are able to be sent in HTML format, this should work (since the HTML format is machform's default, and it gives you the option of the "plain text", which can't be used to display HTML elements.
In the end it depends if the recipient has an email client that supports HTML emails and on their security software set up (e.g. it might block images in Emails).
You as webmaster/admin might be alright with your own system setup, but you can't predict how a user will have their system organised.